You can make it illegal to transact crypto. That would devastate the price and ruin it as a currency. The network would still exist, but it'd be useless as a medium for criminal payments due to the low value.
Outlawing it will make a difference though since it'll make it less likely that victims will pay, meaning the endeavor has a lower success rate.
I put off my 20,000 BTC purchase because it was so sketchy.
There is no significant legitimate commerce taking place using cryptocurrency. By what mechanism would banning converting BTC to USD "financially ruin" the US?
Just last month, China banned the use of crypto by any financial institution. Are you suggesting this will ruin their economy?